In recent years, as electronic components become denser, a circuit board such as a printed wiring board is required to have higher heat dissipativity. As a printed wiring board which is excellent in heat dissipativity, a metal core substrate is known and has already been practically used. A metal core substrate uses a metal having a high thermal conductivity such as aluminum or copper as a core material. In the metal core substrate, an insulating layer such as a resin or the like is formed on both surfaces of the core material, wiring is formed on the surface of the insulating layers, and a semiconductor component, a ceramic component, or the like is mounted thereon. The core material may diffuse over the entire substrate heat from such a heat generating component mounted on the surface of the metal core substrate, thereby suppressing temperature rise of the heat generating component. In such a metal core substrate, aluminum having a low specific gravity is usually used as the core material.
However, while the thermal expansion coefficient of aluminum is about 24 ppm/° C., the thermal expansion coefficient of a ceramic component mounted via a solder joint is about 7 ppm/° C., and thus, a heat cycle test conducted on a circuit board having a ceramic component mounted thereon shows a problem that, due to the thermal expansion coefficient difference between aluminum as the core material and the ceramic component, stress is applied between the circuit board and the ceramic component and a crack develops in the solder joint, with the result that reliability in mounting is not obtained.
As a circuit board which may solve such a problem, a circuit board using carbon fiber reinforced plastics (CFRP) as the core material is disclosed (see, for example, Patent Literature 1).
CFRP is a composite material formed of carbon fiber and a resin. A circuit board using CFRP as the core material has a thermal conductivity which is higher than that of aluminum and has a thermal expansion coefficient which is close to that of the ceramic component, and thus, the circuit board is more excellent in thermal conductivity than an aluminum core substrate and is excellent in reliability in mounting.
When a circuit board is manufactured with the use of CFRP as the core material, CFRP having a copper foil attached to both surfaces thereof is used as the core material to be prepared first (see, for example, Patent Literature 2). As such a double-sided copper-clad substrate, a double-sided copper-clad board is disclosed which is manufactured by sandwiching by copper foils the upper and lower surfaces of a prepreg that is a glass fiber sheet impregnated with an epoxy resin and then carrying out pressure forming (see, for example, Patent Literature 3).